I. Field of the Invention:
This invention relates generally to balloon catheters and more particularly to methods for fabricating a multi-layer balloon composite exhibiting enhanced characteristics attributable to the properties of the individual layers comprising the balloon.
II. Discussion of the Prior Art:
In the Hamlin patent 5,270,086, there is described a method for fabricating multi-layer composite expander members (balloons) for use on angioplasty and other types of balloon catheters. As is pointed out in the Hamlin patent, by selective choice of the materials comprising the individual layers on the multi-layer balloon, the characteristics on the resulting balloon product can be tailored to overcome drawbacks of various polymer materials that have been used in the past in creating single layer balloons. For example, a balloon fabricated from polyethylene terethphalate exhibits excellent burst strength and rupture properties with a desirable low distensibility, but it is difficult to bond such a balloon to a catheter body. By first extruding a tubular parison from a polymeric material having good thermal bonding properties relative to the material of the catheter on which it is used and then co-extruding a layer of polyethylene terethphalate (PET) on that parison, when the composite is subjected to a stretch blow-molding operation in a heated mold, a balloon results that allows ready thermal bonding to a catheter body.
It is also a desirable property of a expander member for an angioplasty catheter that its surface be lubricous. Lubricity may be added to a PET balloon by forming a three-layer tubular parison where, for example, PET is the intermediate layer provided for its high burst strength, low distensibility and known rupture characteristics, an inner polyethylene layer to enhance bondability to a catheter body, and an co-extruded outer layer of a polymer, such as polycaprolactam, which is hydrophilic and exhibits low frictional resistance when passed through the vascular system.
Further information concerning the advantages and properties of co-extruded expander members for medical catheters are set forth in published Patent Cooperation Treaty application W092/19316 and published European application 553,960A1.
The formation of multi-layer balloons using co-extrusion processes often times results in high rejection rates. It is somewhat difficult to maintain uniform layer thicknesses which may result in unacceptable variations in the wall thickness of the resulting balloons once they are formed in a stretch blow-molding operation. Moreover, once parisons for the fabrication of multi-layer expander members are manufactured using a co-extrusion process, the number of layers, their thicknesses and relative positions are fixed. The only way to vary these parameters would be to carry out additional extrusion runs, which is both time-consuming and expensive. In that each extrusion run generates a certain minimum quantity of parison lengths, process iterations cause manufacturing costs to escalate rapidly. It can also be appreciated that the prior art co-extrusion processes are quite inflexible in terms of being able to step through a number of possible multi-layer configurations quickly. Finally, co-extrusion is recognized as a more expensive process than the standard, single polymer extrusion process.
The method of the present invention obviates many of the foregoing drawbacks of the prior art co-extrusion processes for fabricating expander members for angioplasty and other medical catheters. Specifically, following the methods of the present invention materially increases the yield of acceptable multi-layer balloons without materially increasing the manufacturing cost thereof.